[BUG] SAVE command is non-atomic

# Problem:
Writes directly to dump.json with no temp-file-then-rename pattern.

```python
with open(filename, "w") as f:
    json.dump(state, f)
```

# Fix:
```python
tmp = f"{filename}.tmp"
with open(tmp, "w") as f:
    json.dump(state, f)
    f.flush()
    os.fsync(f.fileno())
os.replace(tmp, filename)  # atomic on POSIX and Windows
```